CBQE-927: extend offline_cluster_upgrade with num_stoped_nodes

Change-Id: I3b2e0db646b943cdf08055a66c83c2799bbc2c05
Reviewed-on: http://review.couchbase.org/23653
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>